1. Field of the Invention
The present invention generally relates to a development apparatus and an image forming apparatus, and more particularly relates to a development apparatus and an image forming apparatus such as a copier, a printer, or a facsimile that forms an image using the development apparatus.
2. Description of the Related Art
Some development apparatuses convey toner using a toner carrier such as a toner conveying base plate that causes toner to hop from one electrode to another, instead of using a developing roller or a magnetic carrier that attracts toner.
For example, a development apparatus disclosed in patent document 1 includes a cylindrical toner carrier having multiple electrodes arranged at a certain pitch in the circumferential direction. The electrodes are made up of multiple pairs of adjacent electrodes. Between each pair of adjacent electrodes an alternating electric field is formed. The alternating electric field causes toner on a first electrode of each pair of adjacent electrodes to float and land on a second electrode, or causes toner on the second electrode to float and land on the first electrode (this phenomenon is hereafter called hopping). The toner that continues hopping on the cylindrical toner carrier is conveyed to a development area as the cylindrical toner carrier rotates. In the development area, toner floating near a latent image on a latent image carrier does not come down to the electrodes on the toner carrier, but instead is attracted by electric fields of the latent image and adheres to the latent image. In such a development apparatus, toner hopping on a toner carrier (not adhering to the toner carrier) is used for development instead of toner adhering to a developing roller or a magnetic carrier. This mechanism makes it possible to develop a latent image with a very low voltage, which is not possible with a conventional single- or two-component development method. For example, it becomes possible to make toner adhere to an electrostatic latent image with a potential difference as low as several tens of volts from surrounding non-image areas.
[Patent document 1] Japanese Patent Application Publication No. 3-21967
In the exemplary development apparatus disclosed in patent document 1, an alternating voltage is applied to the electrodes of the toner carrier to form alternating electric fields. Although there is no detailed description in patent document 1 about the alternating voltage to be applied, judging from the configuration of the exemplary development apparatus shown in FIG. 2 in patent document 1, it is assumed that an alternating voltage of one pulse wave, for example, from a 100 V AC power supply widely used in homes is used. Also, as shown in FIG. 16, one of each pair of electrodes is grounded and an alternating voltage with frequency f is applied to the other one.
According to an experiment by the inventors of the present invention, applying an alternating voltage in a manner as described above may cause scumming (smear) on a non-image area (where no latent image is formed) of a latent image carrier. To develop a latent image at high quality with a cloud of toner, it is necessary to form an even toner cloud by causing toner to smoothly hop between electrodes. In the alternating voltage applying method as shown in FIG. 16, the electrostatic force that causes toner to hop between electrodes is generated by the potential difference between a half of the peak-to-peak voltage Vpp at the first electrode to which a pulse wave is applied and 0 volts at the second electrode that is grounded. To increase the electrostatic force and thereby to cause smooth hopping of toner, it is necessary to increase the amplitude of the peak-to-peak voltage Vpp of the pulse wave. However, when the amplitude is increased to the extent necessary to cause smooth hopping, at the upper peak or the lower peak of the pulse wave voltage, electric fields that attract toner on the electrodes to the non-image area of the latent image carrier may be formed for a moment. In other words, when the pulse wave voltage reaches the upper peak or the lower peak, toner may be attracted to the non-image area.